May 7 2018
@Kghbln You're right. I was a little bit confused because in 4.3 $wgPageFormsUseDisplayTitle has changed to true. 
So, changing it to false will vanish the warnings (but you will lose the benefits of using DisplayTitle, wich in my case is not desirable).
Just linking to another report of this problem: https://www.mediawiki.org/wiki/Extension_talk:Page_Forms#Warnings
Dec 9 2017
This problem has returned. Tested in PF 4.2, MW 1.30.0